1. Technical Field
The disclosure relates to illumination, and particularly to an illumination apparatus providing glare prevention.
2. Description of the Related Art
Light emitting diodes' (LEDs) many advantages, such as high luminosity, low operational voltage, low power consumption, compatibility with integrated circuits, easy driving, long term reliability, and environmental friendliness have promoted their wide use as a light source.
Joseph Bielecki et al in IEEE, 23rd IEEE SEMI-THERM Symposium, “Thermal Considerations for LED Components in an Automotive Lamp.” characterize light emitting diodes as one kind of semiconductor device changing current into light of specific wavelength.
Glare, or unwanted occurrence of bright light, can be direct, where the light is emitted from high luminance or glare source and directly on viewers, with the glare source in the same direction or aligned with the observed object, or indirect, where the glare source is not in the direction of observation, and is usually caused by light reflected from a smooth surface. Referring to FIG. 1, when a light source 101 is above a viewer's eyes 102, in the range of 45° to 85° deflecting from the vertical surface 103 through the eyes 102 of the viewer, the light source 101 causes direct glare at the eyes 102.
Currently, many street light illumination apparatus can create direct glare to drivers. Referring to FIG. 2, light from the street light 201 is emitted toward the road surface. Often the radiation range of the street light 201 on the driving direction along axis X exceeds the radiation range of axis Y perpendicular to axis X, which can increases effectively light utilization efficiency of street light 201. However, the radiation range of axis X of the street light 201 is distributed symmetrically about the center of the street light 201. Therefore, both sides of radiation angle of axis X, θ1 and θ2 are equal. Generally, θ1 and θ2 are both equal to 75° producing direct glare to eyes. The radiation angle here is also known as the half-peak side angle. The half-peak side angle is the angle measured at half of maximum light intensity at both sides of the vertical axis of the road surface. Therefore, the half-peak side angle is the included angle of the light of 50% maximum intensity on the plane from the light source and the vertical axis.
Referring to FIG. 3, on a distribution curve of luminous intensity of the street light 201, point A represents light intensity 50% of the maximum between radiation angle 0° to 90°, and point B represents light intensity of the maximum between radiation angle 0° to 90°. Therefore, the radiation angle θ of street light 201 is almost equal to 70°. Thus, the present street light 201 creates direct glare.
What is needed therefore, is an illumination apparatus with glare prevention overcoming the described limitations.